In a case that devices are produced by using a wafer cut out from a semiconductor ingot such as a silicon single crystal, there are many steps from processing of a wafer to formation of a device. There is a step of heat treatment as one of these steps. The heat treatment step is a very important process that is performed for the purpose of formation of a defect-free layer in a surface layer of a wafer, gettering, crystallization, formation of oxide film, impurity diffusion, and such.
As a diffusion furnace used in such heat treatment step of a wafer such as oxidization or impurity diffusion (an apparatus for oxidization and diffusion), along with a diameter of a wafer being larger, there has been mainly used a vertical furnace 20 for heat treatment in which the heat treatment is performed with many wafers W being horizontally supported at a predetermined interval as shown in FIG. 8. Wafers W in the heat treatment furnace 20 can be heated with a heater 24 provided around a reaction chamber 22. During heat treatment, a gas is introduced into the reaction chamber 22 through a gas-introducing duct 26, flows from the upper side to the lower side and is discharged outside from a gas-discharging duct 28. The gas to be used is different according to a purpose of heat treatment. However there are mainly used H2, N2, O2, Ar2, and such. In the case of impurity diffusion, these gasses are also used as a carrier gas of an impurity compound gas.
When a wafer W is subjected to heat treatment by using such a vertical furnace 20 for heat treatment, there is used a vertical boat 11 for heat treatment (hereinafter, occasionally referred to as “a boat for heat treatment”, “a vertical boat”, or simply “a boat”) for horizontally setting many wafers W.
FIG. 9(a) shows a scheme of a general boat 11 for heat treatment. A pair of plate materials (a top plate 16a and a bottom plate 16b) is coupled to both ends of four bar-like (cylindrical) supporting columns (rods) 14. Many slits (grooves) 15 are formed in each of the supporting columns 14 and a convex part between the slits 15 acts as a supporting part 12 for the wafer W. In addition, a boat for heat treatment in which many grooves 15 are formed in the cylindrical supporting columns 14 as described above is generally referred to as a short finger type.
When wafers W are subjected to heat treatment by using such a type of a vertical boat 11 for heat treatment, as shown in FIG. 9(b), peripheral parts of a wafer W are supported at four places by the supporting parts 12 formed at the same height in each of the columns 14, and thereby a wafer W is horizontally supported.
In the case of supporting circumferential part of a wafer W as described above, because wafer's own weight concentrates on the supporting parts, stress caused by this constantly acts. And, if the stress is over a critical shearing stress, a dislocation is generated in the wafer. This dislocation spreads to the degree of a macroscopic size thereby to become a slip by the action of the stress. Because generation of a slip significantly degrades quality of the wafer, it is important to prevent this.
However, generally, a slip becomes remarkably easily generated under an atmosphere at a high temperature. Particularly, along with integration of semiconductor devices being higher, a diameter of a wafer has become larger for improving device yield per one wafer. As a result, wafer's own weight becomes larger, and along with this, the stress acting on a wafer tends to increase, and a slip has become more easily generated in the wafer.
Moreover, due to the enlargement of a size of a wafer, particularly while temperature rising, temperature difference between the central part and the circumferential part of a wafer tends to become larger. A thermal stress generated due to this temperature difference is also one of causes of the above-described generation of a slip.
As means for preventing such a slip from being generated during heat treatment, it is suggested to use an auxiliary jig having a shape of a ring or a circular arc (See Japanese Patent Laid-open (Kokai) Publication No. 6-260438). Concretely, through an auxiliary jig having a shape of a ring or a circular arc which is loaded on supporting parts of each of the columns, a circumferential part of a lower surface of a wafer is supported at the entire circumference or a part thereof with a width of several mm to several tens mm. Moreover, there are suggested a method for supporting the entire backside surface of a wafer, and so forth. By using these methods, there can be obtained an effect of suppressing generation of a slip because a surface for supporting a wafer (a supporting surface) becomes larger and thereby load and such are deconcentrated.
However, in recent years, along with integration of semiconductor devices being higher, there has become emphasized a flaw generated on a backside of a wafer as well as a slip. This is because in a case of using a stepper with a pin chuck system, it has been feared that defocus is caused when a backside flaw of a wafer is placed on a pin of the pin chuck. Therefore, there has been noted a boat for heat treatment or an auxiliary jig which is a type of supporting the circumferential part of a wafer rather than a type of supporting the entire surface of backside of a wafer.
In the case of performing heat treatment on a wafer supported along the circumferential part thereof, it is ideal that the wafer is supported along the entire circumference of a lower surface thereof. However, in a general furnace for heat treatment, a wafer is conveyed by using a jig 31 with a type of scooping up a backside of a wafer as shown in FIG. 10(a) or a jig 32 with a type of sucking a backside of a wafer as shown in FIG. 10(b). Therefore, when a wafer is transferred to an auxiliary jig with a ring form and such, in order that these conveying jigs should not interfere with the ring, there is used a circular arc-shaped auxiliary jig 37 which is provided with a cutout at a part of a ring as shown in FIG. 11(a) or a ring-shaped auxiliary jig 38 which is provided with a step 39 as shown in FIG. 11(b).
The circular arc-shaped auxiliary jig 37 as described above has a merit that the number of wafers for batch heat treatment can be increased because the jig is relatively thin and it is needless to spread pitches of grooves of a boat so much. However, it is difficult to maintain a surface accuracy, and if the heights of both ends of the cutout are different, it is feared to generate a slip.
In the case of the auxiliary jig 38 with a step, the jig can be produced with a relatively high surface accuracy and it is advantageous in terms of reduction of a slip. However, the jig 38 is thicker because of the step 39 existing. Therefore, pitches of grooves of a boat come to spread and the number of wafers for batch heat treatment becomes decreased.
Furthermore, because these auxiliary jigs 37 and 38 are produced by hollowing out a circular plate-shaped member, it is troublesome and very expensive. If there is prepared a complete set of several tens of auxiliary jigs and a vertical boat for batch heat treatment, cost becomes very higher, compared to a general vertical boat for directly supporting a wafer as shown in FIG. 9.
On the other hand, there is also suggested a boat for heat treatment, wherein the circular arc-shaped supporting parts are integrally formed in one cylinder to be a main body of the boat (See Japanese Patent Laid-open (Kokai) Publication No. 5-129214). This boat for heat treatment is that after opening parts (cutouts) for passing a conveying jig and for airflow of a surrounding gas are formed in a cylinder made of SiC and such in the vertical direction (axial direction), horizontal grooves for inserting wafers thereinto and supporting a circumferential part of a lower surface thereof are formed up to the inside of the cylinder in the horizontal direction. Such a boat can support a wafer along a circumferential part of a lower surface thereof without using such a circular arc-shaped auxiliary jig as described above.
However, in the boat that vertical and horizontal grooves are formed in one cylinder as described above, there are problems that it is necessary to form at first a plurality of wide cutouts for passing a conveying jig and for airflow of a surrounding gas, and it is troublesome to form these cutouts in a cylinder. Moreover in the case that chipping or crack is generated even at one point, there is a problem that the entire cylinder becomes wasted, like so, production yield is low, and consequently production cost become higher.
Moreover, there is also a problem that if many cutouts are provided, a slip is easily generated in a wafer.